Unless you require a modern day microcar/city car, such as the smart car or the Toyota Aygo, most car dealerships simply do not deal in microcars. Most classic microcars are no longer in production, unless you want a P50, however, many have become collectors items and it is possible to find them via private sellers or specialist dealers.
The most practical way of gaining knowledge about where to purchase
microcars from is probably to join up with one of the microcar clubs and to attend conventions and meetings. In the UK, the National
Microcar Rally is a group which meets every September to show off their
microcars and discuss all things microcar. Their website has a
comprehensive list of microcar clubs on their links page. Most
are easy and free to join. Although it is possible to find classic
microcars on conventional websites such as Ebay.com, it is exceedingly
Getting involved in clubs and conventions is a lot more realistic
as cars are often bought and sold in this way- more so than on the open
market, it's also a lot more fun, and the members will have bundles of good advice on what to look for and what to avoid, as simple thing like obtaining parts, insurance and finding repairers can become tricky.
Ebay has a great buyers guide for microcars here: Ebay Microcar Buyers Guide